Orthogonal frequency division multiplexing (OFDM) is a relatively well known multiplexing technique for communication systems. OFDM communication systems can be used to provide multiple access communication, where different users are allocated different orthogonal tones within a frequency bandwidth to transmit data at the same time. In an OFDM communication system, the entire bandwidth allocated to the system is divided into orthogonal tones. In particular, for a given symbol duration T available for user data transmission, and a given bandwidth W, the number of available orthogonal tones F is given by WT. The spacing between the orthogonal tones Δ is chosen to be 1/T, thereby making the tones orthogonal. In addition to the symbol duration T which is available for user data transmission, an additional period of time Tc can be used for transmission of a cyclic prefix. The cyclic prefix is prepended to each symbol duration T and is used to compensate for the dispersion introduced by the channel response and by the pulse shaping filter used at the transmitter. Thus, although a total symbol duration of T+Tc is employed for transmitting an OFDM symbol, only the symbol duration T is available for user data transmission and is therefore called an OFDM symbol duration.
In prior OFDM techniques, an OFDM signal is first constructed by a transmitter in the frequency domain by mapping symbols of a constellation to prescribed frequency tones. The signal constructed in the frequency domain is then transformed to the time domain by an inverse discrete Fourier transform (IDFT) or inverse fast Fourier transform (IFFT). A cyclic prefix having duration Tc, as discussed above, is then prepended to the time domain signal corresponding to each symbol duration T resulting in a signal which has the total symbol duration T+Tc for each symbol to be transmitted. The time domain signal including the cyclic prefixes is sampled to obtain the digital signal samples to be transmitted.
In general, symbols of the constellation have a relatively low peak to average ratio property. For example, symbols of a QPSK constellation all have the same amplitude.
However, after being transformed by the IDFT or IFFT, the resultant time domain signal samples are the weighted sum of all the symbols, and therefore generally do not preserve the desirable low peak to average ratio property. In particular, the resulting time domain signal typically has a high peak to average ratio.
Since symbols are mapped to tones in known OFDM transmitters in the frequency domain, symbol recovery is also performed in the frequency domain, e.g., with received signals corresponding to individual tones being mapped back in the frequency domain to individual symbols.
FIG. 1 illustrates an exemplary known OFDM receiver 100. The OFDM receiver 100 includes an antenna 102, tuner 104, A/D converter 106, cyclic prefix discarding circuit 108, FFT circuit 110, training symbol extraction circuit 112, a frequency domain channel estimation circuit 114, a frequency domain channel equalization circuit 118 and decoder 118 coupled together as illustrated in FIG. 1.
Broadcast OFDM signals are received via antenna 102 and then filtered by tuner 104 which outputs a signal which includes the OFDM tones used to transmit symbols. The continuous signal output by the tuner 104 is sampled by A/D converter 106 to generate a digital signal which is then processed by the cyclic prefix discarding circuit 108. Circuit 108 discards the portion Tc of the received signal corresponding to the cyclic prefix. The remaining portion of the signal corresponding to the symbol duration T is supplied to the transform circuit 110, e.g., an FFT or DCT circuit, which converts the time domain signal representing the transmitted symbols into the frequency domain. Training symbol extractor 112 extracts one or more training symbols or pilot tones, i.e., symbols or tones with known transmitted values in the frequency domain, from the received signal. The extracted training symbols/tones are supplied to the frequency domain channel estimation circuit 114. The circuit 114 estimates the effect, in the frequency domain, of the communications channel on the transmitted signals as evidenced by the difference between the between the received training symbol(s) or pilot tone(s) and the expect values. Frequency domain channel equalization circuit 116 receives channel estimation information from circuit 114 and performs channel equalization operations on the frequency domain signal generated by transform circuit 110 to compensate for channel distortions. After channel equalization is performed in the frequency domain, the signal is processed by decoder 118 which maps the frequency domain signal into symbols and/or data.
Existing techniques for implementing OFDM communication systems can be highly inefficient in terms of power utilization due to the relatively high peak to average ratio when compared with other signaling schemes, such as single carrier modulation schemes. As a result, existing OFDM techniques are not well suited for a wireless multiple access communication network with highly mobile users because the high peak to average ratio of the transmitted signal requires a large amount of power at the base station and at the wireless device. The large power requirements result in short battery life and more expensive power amplifiers for handheld wireless communication devices or terminals. Accordingly, it is desirable to provide an OFDM technique which reduces the peak to average ratio of the signal to be transmitted, while simultaneously taking advantage of the larger communication bandwidth offered by an OFDM communication system.